This invention relates to a filter circuit arrangement comprising a reactive element and an electronic reactance circuit, said electronic reactance circuit comprising an amplifier arrangement to the input of which said reactive element is coupled and the output of which is coupled to said reactive element.
It is known (see, for example, the book "Radio Designer's Handbook" (Iliffe) 4th Edition, pages 1156-1160) that the effective value of a reactive element, for example an inductor, can be modified by connecting the anode-cathode path of a valve in parallel with the element, and feeding the voltage across the reactive element to the control grid of the valve via a 90.degree. phase-shifting network, so that the anode-cathode path of the valve carries a signal current which is in phase or anti-phase with the current through the reactive element (if the reactive element is a pure capacitance or a pure inductance). The valve therefore acts as a further reactance (either capacitive or inductive depending on the sense in which the shift occurs in the phase-shifting network) in parallel with the reactive element.
A television receiver circuit normally requires the presence of a so-called "sound trap" filter in its video channel in order to prevent audio signals from affecting the display, the audio signals being conventionally transmitted in the form of frequency-modulation of a subcarrier situated just above the highest transmitted video modulation frequency. This sound trap filter is normally in the form of a so-called "notch" filter tuned to the sound subcarrier frequency (conventionally 6 MHz or 5.5. MHz) and, in order that the frequency response of the video channel should be correct for the video signals, it is normally required that the transmission factor of the notch filter should be substantially constant for the whole of the frequency range of the video signals, i.e. its response should be substantially flat for frequencies up to a frequency just below that at which the notch occurs.
If the notch filter were constituted by a simple series-resonant circuit comprising a single inductor and a single capacitor connected in series across the input signal source impedance the transmission factor of the filter would be liable to start to fall at frequencies well below that at which the notch is situated, i.e. at frequencies within the video signal band, with the result that the higher frequency components of the video output signal from the filter would be liable to be degraded. A higher order filter is therefore desirable. One form of such a higher order filter is shown in FIG. 1 of the accompanying diagrammatic drawings, a second inductor L.sub.2 being included in the input signal path to a conventional series-tuned circuit constituted by a first inductor L.sub.1 and a capacitor C. The values of inductor L.sub.1 and capacitor C are chosen so that the resonant circuit formed thereby resonates at the sound subcarrier frequency, creating a notch in the transmission factor of the filter at this frequency, and the value of inductor L.sub.2 is chosen so that a pole is created in the response of the filter at a frequency just below this frequency. The result is that the transmission factor V/E of the filter varies with frequency f in a manner having the general form shown in FIG. 2, in which f.sub.2 denotes the sound subcarrier frequency, and in which the response which would be obtained at frequencies below f.sub.s if the inductor L.sub.2 were replaced by a simple resistor is shown as a dashed line. The height of the peak occurring in the response just below f.sub.s and the actual frequency at which this occurs can of course be adjusted by varying the value of the inductor L.sub.2 and the "Q" of the circuit, in order to obtain the most satisfactory approximation to the response ideally required.
Nowadays it is highly desirable that as much as possible of television receiver circuitry be constructed in integrated circuit form in the interests of reliability, reproducibility and economy. One form of component which it is very difficult to manufacture in this way is a conventional inductor, and it is desirable that, if possible, any inductive element required be constructed in an alternative way which lends itself more readily to fabrication by integrated circuit techniques.
It is known that an inductor can be replaced by the driving point impedance of a capacitively-loaded gyrator circuit, i.e. the driving point impedance of a capacitively loaded two-port for which the admittance matrix is ideally ##EQU1## A gyrator circuit may be formed, for example, by interconnecting a pair of voltage-controlled current sources, one of which is inverting from input to output and the other of which is non-inverting from input to output, in such manner that the input of each source, is connected in parallel with the output of the other source, each resulting commoned input of one source and output of the other source constituting one port of the gyrator. As another example, a gyrator circuit may be formed by interconnecting a pair of current-controlled voltage sources, one of which is inverting from input to output and the other of which is non-inverting from input to output, in such manner that the input of each source is connected in series with the output of the other source, each resulting series combination of the input of one source and the output of the other source constituting one port of the gyrator. Thus each inductor L.sub.1 and L.sub.2 of FIG. 1 could be replaced by the driving point impedance of a respective capcitively loaded gyrator, thereby making the filter circuit more amenable to fabrication by integrated circuit techniques. However, each gyrator would require a considerable area on the semiconductor chip on which the filter circuit (and ideally as large a proportion as possible of the other parts of the television receiver circuitry) is fabricated, which may result in either the use of a larger chip than is desirable from the point of view of reliable manufacture, or the omission therefrom of other parts of the receiver circuitry which would otherwise be provided thereon.